With the continuous increase of a frequency of a switching power supply, an EMI (Electromagnetic Interference) electromagnetic interference problem is increasingly serious. A traditional method is to add an EMI filter into a system to suppress noise. In order for the filter to suppress the noise to a greatest extent, a designer must select a most appropriate EMI filter according to an impedance mismatch principle. Noise source impedance, load end impedance, and input/output impedance of an EMI filter are key factors for adjusting and configuring performance of the EMI filter according to the impedance mismatch principle.
To resolve the electromagnetic interference problem, a common practice in the industry currently is to select a corresponding type of EMI filter according to high impedance or low impedance of noise source impedance and load impedance. To maximally reduce the EMI in the switching power supply, the designer must select multiple different filters to perform comparison, that is, to adjust an impedance characteristic of an EMI filter from a perspective of the filter, and finally select an EMI filter that is most suitable for the system.
In a process of resolving the electromagnetic interference problem, the inventor finds that at least the following problems exist in the prior art:
In the prior art, an EMI filter is designed according to the impedance mismatch principle, and impedance of the filter is adjusted to achieve maximum mismatch between noise source impedance and input impedance of the EMI filter, and between output impedance of the EMI filter and load impedance. However, the noise source impedance cannot be accurately described currently. In addition, waste of costs and time caused in a type selection comparison test of the EMI filter for the system enables design to be inappropriate and have low efficiency. If over design is selected for the EMI filter to save time and costs, that is, a large margin is reserved when the EMI filter is designed, in this case, a size of the EMI filter becomes very large.